The Stéréo­scope Stan­dard is a fea­ture-rich stere­o­scope. The ocu­lars are attached by means of a bay­o­net mount and are inter­change­able. The stere­o­scope can be equipped with ordi­nary or achro­mat­ic lens­es, avail­able in short, medi­um, or long focal lengths. For the 6 x 13 mod­el, met­al frames were avail­able to allow view­ing of 45 x 107 stere­oviews. The rear sec­tion of the view­er, which holds the ground glass, can be opened and pro­vides space for colour fil­ters to tint the stereoviews.
(from: Com­pendi­um of Stereoscopes)

Fur­ther read­ing: Stere­oscopy His­to­ry Series — Mat­tey (A. Ruiter)